This invention relates to pressure sensors, and more particularly to a pressure sensor for use in a harsh environment.
Sensors for the measurement of various physical parameters such as pressure and temperature often rely on the transmission of strain from an elastic structure (e.g., a diaphragm, bellows, etc.) to a sensing element. In a pressure sensor, the sensing element may be bonded to the elastic structure with a suitable adhesive.
It is also known that the attachment of the sensing element to the elastic structure can be a large source of error if the attachment is not highly stable. In the case of sensors that measure static or very slowly changing parameters, the long term stability of the attachment to the structure is extremely important. A major source of such long term sensor instability is a phenomenon known as xe2x80x9ccreepxe2x80x9d, i.e., change in strain on the sensing element with no change in applied load on the elastic structure, which results in a DC shift or drift error in the sensor signal. It is further known that most attachments transmit a base strain to the sensor through the attachment structure and that a true zero base strain sensitivity is difficult if not impossible to achieve.
One example of a fiber optic based sensor is that described in U.S. patent application Ser. No. 9/205,944 entitled xe2x80x9cTube-Encased Fiber Grating Pressure Sensorxe2x80x9d to T. J. Bailey et al., which is incorporated herein by reference in its entirety. In that case, an optical fiber based sensor is encased within a tube and discloses certain embodiments wherein the sensor is suspended within a fluid. Some examples of such fiber optic sensors include sensors and tubes that are comprised of glass. A disadvantage of such sensors is that the glass is fragile, being brittle and sensitive to cracking. Thus the use of such a sensor in a harsh environment, e.g. where the sensor would be subject to significant levels of shock and vibration, presents a serious threat of damage to the fragile sensor. In certain environments such sensors are subject to shock levels in excess of 100 times the force of gravity (g) and vibration levels of 5 g RMS at frequencies typically ranging from about 10 Hz to about 200 Hz.
However, as discussed hereinbefore, sensor performance is closely tied to attachment techniques and to packaging of the sensor element as well. It is important to package such sensor elements to protect the fragile elements and not impede performance of the sensor in a manner that is reliable and inexpensive to manufacture.
Objects of the present invention include provision of a pressure sensor with minimal base strain and packaging for survival within a harsh environment.
The invention may be used in harsh environments (high temperature, and/or pressure, and/or shock, and/or vibration), such as in oil and/or gas wells, engines, combustion chambers, etc. In one embodiment, the invention may be an all glass fiber optic sensor capable of operating at high pressures ( greater than 15 kpsi) and high temperatures ( greater than 150xc2x0 C.). The invention will also work equally well in other applications independent of the type of environment.
An object of the present invention includes a pressure sensor assembly for measuring a pressure of a first fluid in a harsh environment, wherein the pressure sensor assembly comprises a housing substantially filled with a void free second fluid having a pressure sensor disposed in the fluid within the housing. Attached to the housing is a pressure transmission device in fluid communication with the first fluid transmitting the pressure of the first fluid to the second fluid. The pressure sensor sensing a pressure indicative of the first fluid thereby. It is another object of the present invention to provide a pressure transmission device comprising a bellows, a pressure sensitive valve, an inlet tube having a predetermined inside diameter, or a diaphragm maintaining the second fluid in a void free condition. It is a further object of the present invention to provide for a buffer tube in fluid communication with first fluid.
It is yet another object of the present invention to provide a pressure sensor assembly which further comprises a pressure seal having an orifice disposed in the housing and a signal transmission cable coupled to the sensor extending through the orifice to the outside of the housing. Another object of the present invention provides that the signal transmission cable further comprises a strain relief portion between the seal and the sensing element to provide a near zero base strain for the sensor. It is still another object of the present invention to provide a pressure housing within which the pressure transmission device is disposed, and where the pressure housing is disposed in fluid communication with the first fluid. Another object includes an embodiment wherein the pressure transmission device maintains the second fluid above a predetermined minimum pressure.
It is yet a further object of the present invention to provide for the sensor to float within the second fluid within said housing, and further to provide for a bumper element disposed within the housing limiting movement of the sensor within the housing. It is yet another object to provide for a pair of said bumper elements positioned on the sensor and further to provide a pair of tangs positioned on an inside wall of the housing which cooperate with a pair of grooves positioned on the bumpers to limit the movement of said sensor within said housing. Another object of the invention provides for bumpers to be mounted to the housing and a pair of tangs mounted on the sensor, wherein the tangs cooperate with grooves in the bumpers to limit the movement of the sensor within the housing. It is yet another object of the present invention to provide for a fiber optic based sensor and a fiber optic transmission cable, and further to provide for a Bragg Grating based fiber optic sensor.
It is still further an object of the present invention to provide a pressure sensor for measuring a pressure of a fluid in a harsh environment wherein the pressure sensor comprises a housing substantially filled with the fluid and a pressure sensor disposed in the fluid within the housing and wherein a pressure transmission device disposed in the fluid and coupled to the housing. The pressure transmission device transmits the pressure of the fluid to the pressure sensor and maintains the fluid within the housing in a substantially void free condition and the pressure sensor senses a pressure indicative of fluid.
The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof.